1. Field of the Invention
This invention relates to a thermal transfer printing device and method for effectively printing photo-quality images such as a full-color photograph by use of dye-sublimation inks of different colors and two-gradation images such as character and bar code patterns by use of a thermal wax-transfer ink on a recording medium.
2. Description of the Prior Art
A variety of identification cards such as a credit card, on which two-gradation patterns including characters and codes are printed with a full-color photograph of user's face, are in general use. A printer therefor is required to reproduce various images created by use of a computer or the like on a recording medium such as a plastic card base with high resolution and accuracy.
A thermal transfer printer capable of high-quality printing is relatively simple in handling and structure, and therefore, has been widely used. The thermal transfer printer are grouped into a thermal wax-transfer type and a dye-sublimation type according to the kind of a printing ink for use in printing. The printing ink applied onto a printing ribbon is fused or sublimated by a thermal print head with an array of heating resistance elements while being pressed against the recording medium, thus producing desired images on the recording medium.
Since the dye-sublimation ink comprising a sublimating dye is transferred with subtle grayscales by supplying minutely controlled heating temperature corresponding to the gradation of a given image, it is suitable for producing a pictrial or photorealistic images. When printing a full-color photorealistic image, the inks of at least three primary colors (yellow, magenta, and cyan) are used to represent all colors and gradations by a subtractive color mixture method. The photorealistic printouts having smooth color gradations can be produced by repeating a single color printing three times.
The thermal wax-transfer type ink is suited to printing of a two-gradation (black-and-white) images because of its narrow temperature range of phase transition. That is, the two-gradation images having definite outlines, such as characters and lines, can be represented by pointillistic (black and white) dots produced by applying binary driving currents to the respective heating resistance elements of a thermal print head.
The thermal wax-transfer ink may be applied for producing a photographic image, but the resultant photographic print outputs have too much contrast without much in the way of halftones.
The ink ribbon used in the thermal transfer color printers comprises a long strip of plastic film base and multiple-color inks applied to the respective frames successively defined on the film base. One example of the conventional ink ribbons with three color inks is disclosed in 1 Japanese Patent Application Publication No. SHO 63(1988)-107574.
In the thermal transfer printers disclosed in 2 Japanese Patent Application Publication No. SHO 60(1985)-32472, and 3 Japanese Pat. Appln. Publication No. SHO 60(1985)-154093, four-color printing is performed by use of color inks of the three primary colors plus a black color for printing characters. In 4 U.S. Pat. No. 4,660,051 to Eiichi Sasaki and 5 Japanese Pat. Appln. Pub. No. HEI 3(1991)-67665, a white ink for background printing is further used.
When printouts having a combination of multi-gradation images such as a photograph and monochrome patterns including characters, e.g. an ID card with a photograph of user's face, are produced, a thermal transfer printer capable of monochrome thermal wax-transfer printing and dye-sublimation printing has been used. In 6 Japanese Pat. Appln. Pub. No. HEI 2(1990)-4565, an ink ribbon to which dye-sublimation color inks and a black thermal wax-transfer ink are applied together is adopted so that a multi-gradation image including a photograph is printed with the dye-sublimation color inks and a monochrome image such as characters is printed with the black thermal wax-transfer ink.
Because the dye-sublimation ink is fugitive, a printed surface of the recording medium is generally coated with a transparent protective layer. On that account, an ink ribbon with such a protective layer in addition to the printing inks has been used. (7 Japanese Pat. Appln. Pub. No. SHO 62(1987)-169679, 8 Japanese Pat. Appln. Pub. No. HEI 1(1989)-122485, 9 Japanese Pat. Appln. Pub. No. HEI 1(1989)-127379, and 10 U.S. Pat. No. 4,738,555 to Masayoshi Nagashima).
A printer using a protective layer ribbon separately from a color ink ribbon is disclosed in 11 U.S. Pat. No. 5,266,969, and 12 Japanese Pat. Appln. Pub. No. SHO 61(1986)-154972, for example.
In brief, the prior art thermal transfer printers may be roughly assorted into a printer using a single ink ribbon with multiple color inks (1.about.5), a printer using a single ink ribbon with thermal wax-transfer and dye-sublimation inks (6), a printer using a single ink ribbon with multiple color inks and an ink protective layer (7.about.10), and a printer using an ink ribbon plus a protective layer ribbon (11and 12).
However, the conventional thermal transfer printers as described above involve various problems to be solved. To be more specific, in the case of printing images by use of the single thermal ink ribbon having the successively arranged thermal wax-transfer and dye-sublimation inks by operating a single thermal print head to heat, either of the thermal wax-transfer and dye-sublimation inks is deteriorated in ink-transfer performance because they are different in reaction temperature and transferring property. As a result, satisfactory print outputs cannot be produced. Therefore, in this thermal wax-transfer/dye-sublimation combined type printer, the thermal print head should be operated at different temperatures for severally fusing the thermal wax-transfer ink and sublimating the dye-sublimation ink to transfer the respective inks to the recording medium under suitable conditions. However, such a printer has a common disadvantage of necessitating a complex controlling system for driving the thermal print head so as to be of no practical utility.
In addition, the transferring conditions under which the thermal transfer inks are fused or sublimated and the transferring properties of the inks, which are specified by the behavior of the ink ribbon upon fusing or sublimating of the ink by the thermal print head, are fundamentally different between the thermal wax-transfer ink and dye-sublimation ink. That is, there is a burdensome possibility that the thermal transfer ink fails to be transferred to the recording medium, the ink ribbon cannot be successfully separated from the print head immediately after the ink on the ink ribbon is transferred to the recording medium by the heat generated by the print head, and the ink ribbon per se melts by the heat of the print head.
Meanwhile, the heating resistance elements of the thermal print head essentially show heat hysteresis by which the heat generated rises and lowers with some degree of delay. Furthermore, the heating resistance element being activated to heat is thermally affected by the adjacent heating element out of operation, consequently to cause the heat generated by the active heating element to diffuse to the adjacent resting element.
The heat hysteresis of the heating element of the print head becomes a serious problem particularly when accurate lines having significant meanings in line width, such as a bar code pattern, are printed. That is, the heat generated by the active heating element cannot soon cool down even when a power source for driving the heating element is switched off. Thus, the line printed by moving the thermal print head in the widthwise direction of the line becomes fat compared with printing in the lengthwise direction of the line. To be concrete, in producing a bar code pattern by the thermal transfer printer, a code bar printed by moving the thermal print head in the widthwise direction usually becomes 1.4 times width as that printed by moving the head in the lengthwise direction under the same conditions. This difference in width cannot be neglected.
Moreover, the heat hysteresis of the heating element is a determent to the image quality. For example, when printing a two-gradation image having definite outlines, the outlines of a resultantly produced image become obscure because the heat of the heating element actuated to depict the outlines is absorbed by the adjacent heating element out of operation.
There are some other causes for deterioration of the image quality of the print outputs. One of the causes is inevitable deviations in specific resistance among the heating resistance elements constituting the thermal print head, as it is technically impossible to make the heating elements strictly equal in resistance. Thus, when producing a colored image represented by subtle halftones such as a photograph, the deterioration of image quality becomes conspicuous with increasing the deviations in specific resistance among the heating elements of the print head. In some cases, shading stripes appear as image noises in the print outputs.
The thermal head having a degree of .+-.12% in deviation of resistance among the heating elements is generally permissible. However, in a case of the printer having ability to render 255 grayscales, about 30 grayscales are sacrificed due to the permissible deviation of .+-.12%. As a result, an image is possibly reproduced substantially in the range of only 225 grayscales by use of the thermal print head commonly incorporated in the conventional thermal transfer printer.
Thus, there has been a need for a thermal transfer printer capable of effectively printing high-quality photorealistic images and two-gradation images such as character and bar code patterns on a recording medium.